I. Field of the Invention
The invention relates to crosslinkable polymer-modified asphalt compositions that exhibit improved performance grade specifications for high temperature properties. The invention further relates to a method of improving polymer efficiency in polymer-modified asphalts.
II. Description of the Prior Art
It is known that adding polymer to asphalt improves the high temperature performance grade (PG) of paving asphalt cements as defined under the test methods established by the Strategic Highway Research Program (SHRP). Commonly used polymers include ethylene vinyl acetate (EVA) copolymers and styrene-butadiene-styrene triblock (SBS) copolymer. These polymers may be blended into the asphalt using high shear mix conditions to ensure proper dispersion of the polymer. Of the polymers used, SB or SBS polymers are preferred because of their compatibility with a large number of asphalts.
Blending crosslinkable polymers with paving asphalts produces a change in the viscoelastic behavior of the asphalt. The change in behavior can be attributed to the formation of a network structure as well as to an increase in viscosity and elasticity. The change in viscoelastic properties is related to the amount of polymer added. Network formation occurs above some finite polymer concentration. Thus, higher temperature PG grades are achieved as the polymer, e.g., SBS, concentration in the asphalt increases. However, the SBS is more costly than the asphalt so for economic reasons its use should be kept to a minimum. The amount of SBS polymer needed can be reduced by adding a vulcanizing (crosslinking) agent such as sulfur. The crosslinking agent allows the network structure to form at lower polymer concentration. This improves polymer efficiency, reducing the amount needed to make a specific grade.
U.S. Pat. No. 4,145,322 to Maldonado et al. teaches that addition of sulfur can improve the efficiency of the SB or SBS, decreasing the amount needed to achieve a desired grade. The sulfur is believed to act as a vulcanizing agent yielding crosslinks between the SB or SBS and asphalt.
Typically, with sulfur crosslinking a 3% SBS/asphalt mixture has about the same SHRP high temperature PG grade as an uncrosslinked 5% SBS/asphalt mixture. However, the use of sulfur has some major drawbacks. In particular, adding sulfur to asphalt produces hydrogen sulfide gas as a byproduct. Hydrogen sulfide is extremely poisonous and must be properly removed and treated. This requires the installation and maintenance of special equipment which increases capital and operating costs.
Accordingly, it would be desirable to provide a method for improving crosslinkable polymer high temperature performance efficiency in asphalt without generating hydrogen sulfide as a byproduct, or by at least significantly reducing the amount of sulfur addition required and/or H.sub.2 S produced.